Lubricating oil composition

ABSTRACT

A lubricating oil composition which is excellent in wear resistance, despite its low phosphorus content, low sulfur content and low sulfuric acid ash content, and which exhibits excellent friction reducing effect even when used for a DLC-treated sliding part, is provided by compounding a specific sulfur-containing compound and a specific polar group-containing compound in a base oil.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition and, morespecifically, to a lubricating oil composition which is excellent inwear resistance, despite its low phosphorus content, low sulfur contentand low sulfuric acid ash content, and which exhibits excellent frictionreducing effect even when used for a diamond-like carbon (DLC)-treatedsliding part.

BACKGROUND ART

Current automobile engines use an oxidation catalyst, a three waycatalyst, an NOx occlusion reduction catalyst, a diesel particulatefilter (DPF), etc. for purification of exhaust gases. These exhaust gaspurification devices are known to be adversely affected by metalcomponents, phosphorus components and sulfur components contained in theengine oil. Thus, it is known to be necessary to reduce these componentsin order to prevent the deterioration of these devices.

Various technical developments of automobiles, etc. have been made inrecent years for the purpose of reducing fuel consumption. For example,there may be mentioned a surface treatment technique in sliding partssuch as engines.

A zinc dithiophosphate (Zn-DTP) has been conventionally used over theyears as a wear resisting and antioxidation agent for a lubricating oilfor use in an internal combustion engine such as a gasoline engine, adiesel engine or a gas engine and is now still accepted as an importantessential additive for such a lubricating oil for internal combustionengines.

The zinc dithiophosphate, which generates sulfuric acid and phosphoricacid upon being decomposed, however, may consume basic compoundscontained in the engine oil and accelerate the deterioration of thelubricant oil with the result that oil change intervals are extremelyshort. Additionally, the zinc dithiophosphate tends to form a sludgewhen subjected to high temperature conditions and to cause deteriorationof the detergency inside an engine. Moreover, the zinc dithiophosphatewhich contains, in the molecule thereof, a large amount of phosphorusand sulfur components in addition to a metal (zinc) component isconsidered to cause an adverse influence on an exhaust gas purifyingdevice. In this circumstance, it is desired to develop a lubricating oilcomposition which excels in a wear resistance without use of the zincdithiophosphate.

In the circumstance in which development of techniques for surfacetreatment of sliding parts is being made as described above, there is ademand for further improvement of lubricating oil compositions. Forexample, when a conventional lubricating oil is used for a sliding partwhich has been subjected to a DLC treatment, there is often a case inwhich an expected friction reducing effect is not achievable. Thus,there is a demand for a lubricating oil composition which exhibitsexcellent friction reducing effect, even when used for DLC-treatedsliding parts, to achieve further lower-fuel consumption.

With a view toward solving these problems, various lubricating oiladditives and lubricating oil compositions have been hitherto proposed.For example, Patent Documents 1 to 3 disclose lubricating oil additivesand lubricating oil compositions which contain as a principle componenta disulfide compound having a specific structure. Patent Document 4discloses an engine oil which is alleged to be able to reduce sulfur andphosphorus that serve as a poisoning substance of reduction catalystsand to excel in friction reducing performance. Patent Document 5discloses a low friction sliding mechanism which has a sliding surfaceusing diamond and which is provided with a lubricating oil compositioncontaining a specific additive.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP-A-2004-262964-   Patent Document 2: JP-A-2004-262965-   Patent Document 3: JP-A-2008-056876-   Patent Document 4: JP-A-2007-131792-   Patent Document 5: JP-A-2008-56735

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Although development of various lubricating oil additives andlubricating oil compositions has been thus far been made as describedabove, the lubricating oil compositions disclosed in the above documentsare not fully satisfactory when taking into consideration thatlubricating oils are generally required to satisfy various performances,such as performance against catalytic poisoning, wear resistance andfriction reducing effect, at the same time. In particular, it has beendifficult to provide a lubricating oil composition, which exhibitsperformances comparable to or better than those of the conventionalones, without using zinc dithiophosphate which is a very effectiveadditive for improving wear resistance and oxidation resistance.

The present invention has been made in view of the foregoingcircumstances and is aimed at the provision of a lubricating oilcomposition which is excellent in wear resistance, despite its lowphosphorus content, low sulfur content and low sulfuric acid ashcontent, and which exhibits excellent friction reducing effect even whenused for a DLC-treated sliding part.

Means for Solving the Problem

The present inventors have made an earnest study and found that theabove-described object can be achieved by using a specificsulfur-containing compound in combination with a specific polargroup-containing compound. The present invention has been completedbased on such finding.

Namely, the present invention provides:

<1> A lubricating oil composition comprising

a base oil, (A) at least one selected from sulfur-containing compoundsrepresented by the general formulas (I) and (II) shown below, and (B) apolar group-containing compound which has at least one polar groupselected from amino groups, amide groups and a hydroxyl group and whichhas a C₃ to C₂₄ alkyl group;

(in the above formulas, R¹ to R¹² each independently represent ahydrogen atom; a hydrocarbon group selected from alkyl groups,cycloalkyl groups, alkenyl groups, cycloalkenyl groups and aryl groups;or a hetero atom-containing group having an atom which is selected froman oxygen atom, a nitrogen atom and a sulfur atom and which is containedin the above hydrocarbon group; Ys each independently represent adivalent group selected from —O—, —S—, —SO—, —SO₂—, —(C═O)O—, —(C═O)NH—,—O(C═O)NH—, —C(═O)—, —N(H)—, —NHCONH—, —N═N—, —NH—C(═NH)—NH—, —S—C(═O)—,—NH—C(═S)— and —NH—C(═S)—NH—; x represents an integer of 1 to 3; and nseach independently represent an integer of 1 to 5);

<2> The lubricating oil composition according to above <1>, wherein (B)the polar group-containing compound, which has at least one polar groupsselected from amino groups, amide groups and a hydroxyl group and whichhas a C₃ to C₂₄ alkyl group, is at least one compound selected from thegroup consisting of glycerol partial esters of fatty acids, glycerolmonoether compounds, amine compounds and amide compounds;<3> The lubricating oil composition according to above <1>, wherein (B)the polar group-containing compound, which has at least one polar groupsselected from amino groups, amide groups and a hydroxyl group and whichhas a C₃ to C₂₄ alkyl group, is a glycerol monoester of a fatty acidrepresented by the general formula (III) or (IV) shown below or aglycerol monoether compound represented by the general formula (V) or(VI) shown below:

wherein R¹³ and R¹⁴ each independently represent a C₃ to C₂₄ alkylgroup;<4> The lubricating oil composition according to about <1>, wherein (B)the polar group-containing compound, which has at least one polar groupsselected from amino groups, amide groups and a hydroxyl group and whichhas a C₃ to C₂₄ alkyl group, is an amine compound represented by thegeneral formula (VII) shown below or an amide compound represented bythe general formula (VIII) shown below:

wherein R¹⁵ and R¹⁷ each independently represent a C₃ to C₂₄ alkylgroup, and R¹⁶ and R¹⁸ each independently represent a hydrogen atom or agroup having a hydroxyl group substituted for a terminal hydrogen atomof a straight chained C₂ to C₄ alkyl group;<5> The lubricating oil composition according to any one of above <1> to<4>, wherein the lubricating oil composition has a phosphorus content of0.5% by mass or less and a sulfuric acid ash content of 0.6% by mass orless, each based on the total mass of the lubricating oil composition;<6> The lubricating oil composition according to any one of above <1> to<5>, wherein the lubricating oil composition has a phosphorus content of0% by mass and a sulfuric acid ash content of 0.1% by mass or less, eachbased on the total mass of the lubricating oil composition; and<7> The lubricating oil composition according to any one of above <1> to<6>, wherein the lubricating oil composition is used for a sliding partwhich has been treated with diamond-like carbon (DLC).

Effect of the Invention

According to the present invention, there is provided a lubricating oilcomposition which is excellent in wear resistance, despite its lowphosphorus content, low sulfur content and a low sulfuric acid ashcontent, and which exhibits excellent friction reducing effect even whenused for a DLC-treated sliding part.

EMBODIMENTS OF THE INVENTION

The lubricating oil composition of the present invention ischaracterized in that a specific sulfur-containing compound and aspecific polar group-containing compound are compounded in a base oil.

Base Oil:

The base oil used in the present invention is not specifically limitedand may be appropriately selected from any mineral oils and syntheticoils that are conventionally used as a base oil for lubricant oils.

Examples of the mineral oils include those which are obtained bysubjecting a lube-oil distillate (which is obtained by vacuumdistillation of an atmospheric residue produced by atmosphericdistillation of a crude oil) to one or more refining treatments such assolvent deasphalting, solvent extraction, hydrocracking, solventdewaxing, catalytic dewaxing and hydrorefining, and those which areproduced by isomerizing waxes or GTL waxes.

Examples of the synthetic oils include polybutene, polyolefins (α-olefinhomopolymers and copolymers (such as ethylene-α-olefin copolymers)),various esters (such as polyol esters, dibasic acid esters andphosphoric acid esters), various ethers (such as polyphenyl ethers),polyglycols, alkyl benzenes and alkyl naphthalenes. Among thesesynthetic oils, particularly preferred are polyolefins and polyolesters.

In the present invention, the above mineral oils may be used alone or incombination of two or more thereof as the base oil. Also, the abovesynthetic oils may be used alone or in combination of two or morethereof. Further, one or more mineral oils may be used in combinationwith one or more synthetic oils.

The viscosity of the base oil is not specifically limited. However, itis preferred that the base oil have a kinematic viscosity at 100° C. of2 to 30 mm²/s, more preferably 3 to 15 mm²/s, still more preferably 4 to10 mm²/s.

When the kinematic viscosity at 100° C. is 2 mm²/s or more, anevaporation loss is small. When the kinematic viscosity is 30 mm²/s orless, a power loss by viscosity resistance can be suppressed so that afuel consumption improving effect is obtainable.

It is also preferred that the base oil have a % C_(A) value of 3.0 orless as measured by ring analysis and a sulfur content of 50 ppm by massor less. As used herein, the term “% C_(A) value as measured by ringanalysis” means a proportion (percentage) of an aromatic component whichis calculated by the n-d-M ring analysis method. The sulfur content asused herein means the value as measured according to JIS K 2541.

The base oil having a % C_(A) value of 3.0 or less and a sulfur contentof 50 ppm by mass or less exhibits good oxidation stability and can givea lubricant oil composition that can suppress an increase of the acidvalue and formation of a sludge. The % C_(A) value is more preferably1.0 or less, still more preferably 0.5 or less. The sulfur content ismore preferably 30 ppm by mass or less.

It is further preferred that the base oil have a viscosity index of 70or more, more preferably 100 or more, still more preferably 120 or more.When the viscosity index of the base oil is 70 or more, a change inviscosity of the base oil by a change in temperature is small.

Sulfur-Containing Compound:

The lubricating oil composition of the present invention contains asulfur-containing compound represented by the following general formula(I) or (II):

In the general formulas (I) and (II), R¹ to R¹² each independentlyrepresent a hydrogen atom; a hydrocarbon group selected from alkylgroups, cycloalkyl groups, alkenyl groups, cycloalkenyl groups and arylgroups; or a hetero atom-containing group having an atom which isselected from an oxygen atom, a nitrogen atom and a sulfur atom andwhich is contained in the above hydrocarbon group.

The alkyl group represented by R¹ to R¹² is preferably a C₁ to C₃₀ alkylgroup, preferably a C₁ to C₂₄ alkyl group. Specific examples of thealkyl group include n-butyl groups, isobutyl groups, sec-butyl groups,tert-butyl groups, various hexyl groups, various octyl groups, variousdecyl groups, various dodecyl groups, various tetradecyl groups, varioushexadecyl groups, various octadecyl groups and various icosyl groups.The alkyl group may be substituted with an aromatic group, examples ofwhich include a benzyl group and a phenethyl group.

The cycloalkyl group represented by R¹ to R¹² is preferably a C₃ to C₃₀cycloalkyl group, more preferably a C₃ to C₂₄ cycloalkyl group. Specificexamples of the cycloalkyl group include a cyclopropyl group, acyclopentyl group, a cyclohexyl group, a methylcyclopentyl group, adimethylcyclopentyl group, a methylethylcyclopentyl group, adiethylcyclopentyl group, a methylcyclohexyl group, a dimethylcyclohexylgroup, a methylethylcyclohexyl group and a diethylcyclohexyl group. Thecycloalkyl group may be substituted with an aromatic group, examples ofwhich include a phenylcyclopentyl group and a phenylcyclohexyl group.

The alkenyl group represented by R¹ to R¹² is preferably a C₂ to C₃₀alkenyl group, more preferably a C₂ to C₂₄ alkenyl group. Specificexamples of the alkenyl group include a vinyl group, an allyl group, a1-butenyl group, a 2-butenyl group, a 3-butenyl group, 1-methylvinylgroup, a 1-methylallyl group, a 1,1-dimethylallyl group, a 2-methylallylgroup, a nonenyl group, a decenyl group and an octadecenyl group. Thealkenyl group may be substituted with an aromatic group.

The cycloalkenyl group represented by R¹ to R¹² is preferably a C₃ toC₃₀ cycloalkenyl group, more preferably a C₃ to C₂₄ cycloalkenyl group.Specific examples of the cycloalkenyl group include a cyclobutenyl groupand a methylcyclobutenyl group. The cycloalkenyl group may besubstituted with an aromatic group.

The aryl group represented by R¹ to R¹² is preferably a C₆ to C₃₀ arylgroup, more preferably a C₆ to C₂₄ aryl group. Specific examples of thearyl group include a phenyl group, a tolyl group, a xylyl group, anaphthyl group, a butylphenyl group, an octylphenyl group and anonylphenyl group.

In the general formulas (I) and (II), Ys each independently represent adivalent group selected from —O—, —S—, —SO—, —SO₂—, —(C═O)O—, —(C═O)NH—,—O(C═O)NH—, —C(═O)—, —N(H)—, —NHCONH—, —N═N—, —NH—C(═NH)—NH—, —S—C(═O)—,—NH—C(═S)— and —NH—C(═S)—NH—.

In the general formulas (I) and (II), x is an integer of 1 to 3,preferably 2, and ns each independently represent an integer of 1 to 5,preferably 1 or 2.

As the sulfur-containing compound represented by the general formula(I), there may be mentioned, for example, compounds of the formulasshown below:

The following compounds are also examples of the compound represented bythe general formula (I), i.e. such examples include:bis(methoxycarbonylmethyl)disulfide, bis(ethoxycarbonylmethyl)disulfide,bis(n-propoxycarbonylmethyl)disulfide,bis(isopropoxycarbonylmethyl)disulfide,bis(n-butoxycarbonylmethyl)disulfide,bis(n-octoxycarbonylmethyl)disulfide,bis(n-dodecyloxycarbonylmethyl)disulfide,bis(cyclopropoxycarbonylmethyl)disulfide,1,1-bis(2-methoxycarbonylethyl)disulfide,1,1-bis(3-methoxycarbonyl-n-propyl)disulfide,1,1-bis(4-methoxycarbonyl-n-butyl)disulfide,1,1-bis(2-ethoxycarbonylethyl)disulfide,1,1-bis(2-n-propoxycarbonylethyl)disulfide,1,1-bis(2-isopropoxycarbonylethyl)disulfide and1,1-bis(2-cyclopropoxycarbonylethyl)disulfide.

Specific examples of the compound represented by the general formula(II) include tetramethyl dithiomalate, tetraethyl dithiomalate,tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate, tetra-1-butyldithiomalate, tetra-2-butyl dithiomalate, tetraisobutyl dithiomalate,tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate,tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyldithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate,tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate,tetrabenzyl dithiomalate, tetra-α-(methyl)benzyl dithiomalate,tetra-α,α-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyldithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate,tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyldithiomalate, tetra-1-(2-butoxybutoxy)ethyl dithiomalate andtetra-1-(2-phenoxy)ethyl dithiomalate.

In the present invention the sulfur-containing compounds represented bythe general formulas (I) or (II) may be used singly or as a mixture oftwo or more thereof. The compounding amount of the sulfur-containingcompound is preferably 0.01% to 5.0% by mass, more preferably 0.1% to2.0% by mass, based on the total mass of the composition. When thecompounding amount is 0.01% by mass or more, a sufficient wearresistance is obtainable. When the compounding amount exceeds 5.0% bymass, there is a possibility that the effect proportional to the amountadded is not obtainable.

Polar Group-Containing Compound:

The lubricating oil composition of the present invention contains apolar group-containing compound which has at least one polar groupselected from amino groups, amide groups and a hydroxyl group and whichhas an alkyl group having a specific number of carbon atoms

The polar group-containing compound used in the present invention is acompound having a C₃ to C₂₄, preferably C₈ to C₂₀ alkyl group. When thenumber of carbon atoms is less than 3, the solubility of the compound islow. Although better friction reducing effect is generally obtainable asthe number of carbon atoms increases, an effect proportional to theincreased number of carbon atoms is hardly obtainable, when the numberof carbon atoms exceeds 24.

As the polar group-containing compound, there may be mentioned, forexample, those which are selected from glycerol partial esters of fattyacids, glycerol monoether compounds, amine compounds and amide compoundsand which have the above-described alkyl group.

The glycerol partial ester of a fatty acid may be, for example, acompound obtained by reaction of glycerol with a fatty acid. Examples ofthe fatty acid include acetic acid, propionic acid, butanoic acid(butyric acid), pentanoic acid (valeric acid), isopentanoic acid(isovaleric acid), hexanoic acid (caproic acid), heptanoic acid,isoheptanoic acid, octanoic acid (caprylic acid), 2-ethylhexanoic acid,isooctanoic acid, nonanoic acid (pelargonic acid), isononanoic acid,decanoic acid (capric acid), isodecanoic acid, undecanoic acid,isoundecanoic acid, dodecanoic acid (lauric acid), isododecanoic acid,tridecanoic acid, isotridecanoic acid, tetradecanoic acid (myristicacid), hexadecanoic acid (palmitic acid), octadecanoic acid (stearicacid), isostearic acid, eicosanoic acid (arachidic acid), docosanoicacid (behenic acid), tetracosanoic acid (lignoceric acid), hexacosanoicacid (cerotic acid), octacosanoic acid (montanic acid), 10-undecenoicacid, zomaric acid, oleic acid, elaidic acid, linoleic acid, linolenicacid, gadoleic acid, erucic acid and selacholeic acid. A mixed fattyacid obtainable from natural fats and oils may also be used. Among thesefatty acids, C₁₀ to C₁₈ fatty acids are preferred, C₁₀ to C₁₂ saturatedfatty acids are more preferred, oleic acid and elaidic acid are furtherpreferred and oleic acid is most preferred from the standpoint of thefriction reducing effect of the lubricant oil.

As the glycerol partial ester of a fatty acid, there may be mentioned,for example, glycerol monoesters of fatty acids represented by thegeneral formulas (III) and (IV) shown below. These compounds may beobtained by, for example, direct esterification between a fatty acid andglycerol or interesterification of a fat and oil with glycerol.

In the general formulas (III) and (IV), R¹³s each independentlyrepresent a C₃ to C₂₄ alkyl group. In the present invention, theglycerol monoesters of fatty acids represented by the general formulas(III) and (IV) may be used singly or as a mixture thereof. In use ofthese glycerol monoesters of fatty acids, glycerol diesters and/ortriesters of fatty acids may be contained therein.

The glycerol monoether compounds may be, for example, compounds obtainedby reaction of glycerol with an aliphatic alcohol. Examples of thealiphatic alcohol include propanol, butanol, oleyl alcohol and stearylalcohol. Above all, from the standpoint of friction reducing effect ofthe lubricant oil, oleyl alcohol and stearyl alcohol are preferred, andoleyl alcohol is particularly preferred.

As the glycerol monoether compound, there may be mentioned, for example,glycerol monoether compound represented by the general formulas (V) and(VI) shown below:

In the general formulas (V) and (VI), R¹⁴s each independently representa C₃ to C₂₄ alkyl group. In the present invention, the glycerolmonoether compounds represented by the general formula (V) and (VI) maybe used singly or as a mixture thereof. In use of these glycerolmonoether compounds, glycerol diethers and/or glycerol triethers may becontained therein.

Examples of the amine compound include alkylamine compounds andalkanolamine compounds. The alkyl group of the alkylamine compound maybe the alkyl group which is contained in the above-described fattyacids. As the amine compound, there may be mentioned, for example, thoserepresented by the following general formula (VII):

In the general formula (VII), R¹⁵ represents a C₃ to C₂₄ alkyl group,and R¹⁶s each independently represent a hydrogen atom or a group havinga hydroxyl group substituted for a terminal hydrogen atom of a straightchained C₂ to C₄ alkyl group (such as —(CH₂)₂—OH). In the presentinvention, the amine compounds represented by the general formula (VII)may be used singly or as a mixture thereof.

Specific examples of the amine compounds include monoethanolamine,diethanolamine, triethanolamine, N-methylethanolamine,N,N-dimethylethanolamine, N-ethylethanolamine, N,N-diethylethanolamine,N-isopropylethanolamine, N,N-diisopropylethanolamine,monoisopropanolamine, diisopropanolamine, triisopropanolamine,N-methylisopropanolamine, N,N-dimethylisopropanolamine,N-ethylisopropanolamine, N,N-diethylisopropanolamine,N-isopropylisopropanolamine, N,N-diisopropylisopropanolamine,mono-n-propanolamine, di-n-propanolamine, tri-n-propanolamine,N-methyl-n-propanolamine, N,N-dimethyl-n-propanolamine,N-ethyl-n-propanolamine, N,N-diethyl-n-propanolamine,N-isopropyl-n-propanolamine, N,N-diisopropyl-n-propanolamine,monobutanolamine, dibutanolamine, tributanolamine, N-methylbutanolamine,N,N-dimethylbutanolamine, N-ethylbutanolamine, N,N-diethylbutanolamine,N-isopropylbutanolamine and N,N-diisopropylbutanolamine.

As the amide compound, there may be mentioned compounds obtainable byreaction of mono- to tetravalent carboxylic acid with an alkylamine oran alkanolamine.

The monovalent carboxylic acid may have an alkyl group such a hexylgroup, a heptyl group, an octyl group, a nonyl group, a decyl group, anundecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, apentadecyl group, a hexadecyl group, a heptadecyl group, an octadecylgroup, a nonadecyl group, an icosyl group, a pentaicosyl group, adocosyl group, a tricosyl group, a tetracosyl group, a pentacosyl group,a hexacosyl group, a heptacosyl group, an octacosyl group, a nonacosylgroup and a triacontyl group.

Examples of the monovalent carboxylic acid include caproic acid,caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid,stearic acid, arachidic acid, behenic acid and lignoceric acid. Examplesof the di- to tetravalent carboxylic acids include polycarboxylic acidssuch as oxalic acid, phthalic acid, trimellitic acid and pyromelliticacid.

As the amide compound, there may be mentioned a compound represented bythe following general formula (VIII).

In the general formula (VIII), R¹⁷ represents a C₃ to C₂₄ alkyl group,and R¹⁸s each independently represent a hydrogen atom or a group havinga hydroxyl group substituted for a terminal hydrogen atom of a straightchained C₂ to C₄ alkyl group (such as —(CH₂)₂—OH). In the presentinvention, the amide compounds represented by the general formula (VIII)may be used singly or as a mixture thereof.

Specific examples of the amide compound include oleic acidmonoethanolamide, oleic acid diethanolamide, oleic acidmonopropanolamide and oleic acid dipropanolamide.

The polar group-containing compound used in the present invention mayalso be a compound obtainable by reaction of the above-described polargroup-containing compound with a molybdenum compound. Examples of themolybdenum compound include molybdenum oxide, molybdenum halide andmolybdic acid. In this reaction, the molybdenum compound is preferablyused in a molar ratio of 0.01 to 10 moles, more preferably 0.05 to 5moles, per mole of the polar group-containing compound.

The reaction may be carried out using a solvent, for example an organicsolvent such as a hydrocarbon oil, hexane, heptane, octane, toluene andxylene.

The reaction temperature for the above reaction is not specificallylimited but is preferably 50 to 250° C., more preferably 100 to 200° C.

The polar group-containing compound used in the present invention mayalso be a compound obtainable by reaction of the above-described polargroup-containing compound with a boron compound. Examples of the boroncompound include boron oxide, a boron halide, boric acid, boricanhydride and an ester of boric acid. In this reaction, the boroncompound is preferably used in a molar ratio of 0.01 to 10 moles, morepreferably 0.05 to 5 moles, per mole of the polar group-containingcompound.

The reaction may be carried out using a solvent, for example an organicsolvent such as a hydrocarbon oil, hexane, heptane, octane, toluene andxylene.

The reaction temperature for the above reaction is not specificallylimited but is preferably 50 to 250° C., more preferably 100 to 200° C.

In the present invention, the polar group-containing compounds may beused singly or as a mixture thereof. The compounding amount of the polargroup-containing compound is preferably 0.01% to 5.0% by mass, morepreferably 0.1% to 2.0% by mass, based on the total mass of thecomposition. When the compounding amount is 0.01% by mass or more, asufficient friction reducing effect is obtainable. When the compoundingamount is 5.0% by mass or more, there is a possibility that undissolvedresidues may be present.

In the lubricating oil composition of the present invention, acustomarily employed additive may be compounded as long as the effectthereof is not adversely affected. Examples of the additive include anantioxidant, an ashless dispersant, a metallic detergent, a viscosityindex improver, a pour point depressant, a metal deactivator, a rustinhibitor and a defoaming agent.

The above-mentioned antioxidant is preferably a phosphorus-freeantioxidant. Examples of the phosphorus-free antioxidant include aphenol-based antioxidant, an amine-based antioxidant, a molybdenum/aminecomplex-based antioxidant and a sulfur-based antioxidant.

Specific examples of the phenol-based antioxidant include4,4′-methylenebis(2,6-di-t-butylphenol), 4,4′-bis(2,6-di-t-butylphenol),4,4′-bis(2-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),4,4′-isopropylidenebis(2,6-di-t-butylphenol),2,2′-methylenebis(4-methyl-6-nonylphenol),2,2′-isobutylidenebis(4,6-dimethylphenol),2,2′-methylenebis(4-methyl-6-cyclohexylphenol),2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol,2,4-dimethyl-6-t-butylphenol, 2,6-di-t-amyl-p-cresol,2,6-di-t-butyl-4-(N,N′-dimethylaminomethylphenol),4,4′-thiobis(2-methyl-6-t-butylphenol),4,4′-thiobis(3-methyl-6-t-butylphenol),2,2′-thiobis(4-methyl-6-t-butylphenol),bis(3-methyl-4-hydroxy-5-t-butylbenzyl)sulfide,bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfide,n-octyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate,n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, and2,2′-thio[diethyl-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate].

Above all, especially preferred are bisphenol-based antioxidants andester group-containing phenol-based antioxidants.

Specific examples of the amine-based antioxidant includemonoalkyldiphenylamines such as monooctyldiphenylamine andmonononyldiphenylamine; dialkyldiphenylamines such as4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine,4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine,4,4′-dioctyldiphenylamine and 4,4′-dinonyldiphenylamine;polyalkyldiphenylamines such as tetrabutyldiphenylamine,tetrahexyldiphenylamine, tetraoctyldiphenylamine andtetranonyldiphenylamine; α-naphthylamine; phenyl-α-naphthylamine; andalkyl-substituted phenyl-α-naphthylamines such asbutylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine,hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine,octylphenyl-α-naphthylamine and nonylphenyl-α-naphthylamine.

Above all, the dialkyldiphenylamine-based and naphthylamines-basedantioxidants are preferred.

As the molybdenum/amine complex-based antioxidants, there may bementioned, for example, hexavalent molybdenum compounds. Specificexamples of such compounds include those which are obtained by reactingmolybdenum trioxide and/or molybdic acid with an amine compound andthose which are obtained by the production method described inJP-A-2003-252887.

The amine compound to be reacted with the hexavalent molybdenum compoundis not particularly limited, and there may be mentioned monoamines,diamines, polyamines and alkanol amines. Specific examples of the aminecompound include alkyl amines having an C₁ to C₃₀ alkyl group or groups(the alkyl group may be either linear or branched) such as methylamine,ethylamine, dimethylamine, diethylamine, methylethylamine andmethylpropylamine; alkenyl amines containing a C₂ to C₃₀ alkenyl groupor groups (the alkenyl group may be linear or branched) such as ethenylamine, propenyl amine, butenyl amine, octenyl amine and oleyl amine;alkanol amines containing a C₁ to C₃₀ alkanol group or groups (thealkanol group may be linear or branched) such as methanol amine, ethanolamine, methanol ethanol amine and methanol propanol amine; alkylenediamines containing a C₁ to C₃₀ alkylene group or groups such asmethylenediamine, ethylenediamine, propylenediamine and butylenediamine;polyamines such as diethylenetriamine, triethylenetetramine,tetraethylenepentamine and pentaethylenehexamine; compounds, such asundecyldiethylamine, undecyldiethanol amine, dodecyldipropanol amine,oleyldiethanol amine, oleylpropylenediamine andstearyltertraethylenepentamine, which are obtained by furtherintroducing a C₈ to C₂₀ alkyl or alkenyl group into the abovemonoamines, diamines or polyamines; heterocyclic compounds such asimidazoline; alkyleneoxide adducts of these compounds; and mixtures ofthese compounds.

In addition, as the molybdenum/amine complex-based antioxidants, theremay be mentioned, for example, sulfur-containing molybdenum complexes ofsuccinic imide as described in JP-B-3-22438 and JP-A-2004-2866.

As the sulfur-based antioxidant, there may be mentioned, for example,phenothiazine, pentaerythritol-tetrakis-(3-lauryl thiopropionate),didodecyl sulfide, dioctadecyl sulfide, didodecyl thiodipropionate,dioctadecyl thiodipropionate, dimyristyl thiodipropionate,dodecyloctadecyl thiodipropionate and 2-mercaptobenzoimidazole.

Among these antioxidants, from the standpoint of reducing a metalcontent and a sulfur content, phenol-based antioxidants and amine-basedantioxidants are preferred. The above antioxidants may be used singly oras a mixture of two or more thereof. From the standpoint of improvedoxidation stability, a mixture of one or more kinds of phenol-basedantioxidants and one or more kinds of amine-based oxidants is preferablyused.

The compounding amount of the antioxidant is generally 0.1% to 5% bymass, more preferably from 0.1% to 3% by mass, based on the total massof the composition.

As the above-mentioned ashless dispersant, there may be used any ashlessdispersant which is generally used for lubricant oils. Examples of theashless dispersant include a mono-type succinimide compound representedby the following general formula (IX) or a bis-type succinimide compoundrepresented by the following general formula (X):

In the above general formulas (IX) and (X), R¹⁹, R²¹ and R²⁴ eachrepresent an alkenyl or alkyl group having a number-average molecularweight of 500 to 4,000. The groups R²¹ and R²⁴ may be the same ordifferent. The number-average molecular weight of R¹⁹, R²¹ and R²⁴ ispreferably from 1,000 to 4,000.

When the number-average molecular weight of R¹⁹, R²¹ and R²⁴ is 500 ormore, the solubility of the compound in the base oil is good. When thenumber-average molecular weight is 4,000 or less, there is no fear ofdeterioration of the dispersancy.

In the formulas, R²⁰, R²² and R²³ each represent a C₂ to C₅ alkylenegroup. The groups R²² and R²³ may be the same or different. The symbol ris an integer of 1 to 10, s is 0 or an integer of 1 to 10. The symbol ris preferably 2 to 5, more preferably 3 or 4. When r is 1 or more, gooddispersancy may be obtained. When r is 10 or less, the compound exhibitsgood solubility in the base oil.

Further, in the general formula (X), s is preferably 1 to 4, morepreferably 2 or 3. The symbol s that lies within the above-specifiedrange is preferred for reasons of the dispersancy and solubility in thebase oil.

Examples of the alkenyl group include a polybutenyl group, apolyisobutenyl group and an ethylene-propylene copolymer group. Examplesof the alkyl group include those which are obtained by hydrogenatingthese alkenyl groups. Typical examples of the suitable alkenyl groupinclude a polybutenyl group and a polyisobutenyl group. The polybutenylgroup may be obtained by polymerizing a mixture of 1-butene andisobutene, or high-purity isobutene.

Typical examples of the suitable alkyl group include those which areobtained by hydrogenating a polybutenyl group and a polyisobutenylgroup.

The above alkenylsuccinimide compound or alkylsuccinimide compound maybe generally produced by reacting a polyamine with an alkenylsuccinicanhydride obtained by reaction of a polyolefin with maleic anhydride, orwith an alkylsuccinic anhydride, obtained by hydrogenating thealkenylsuccinic anhydride. Also, the above mono-type succinimidecompound or bis-type succinimide compound may be produced by varying areaction ratio between the alkenylsuccinic anhydride or alkylsuccinicanhydride and the polyamine.

As an olefin monomer from which the above polyolefin is formed, theremay be used a C₂ to C₈ α-olefin or a mixture of two or more thereof.Among them, a mixture of isobutene and butene-1 may be suitably used.

Examples of the polyamine include primary diamines such asethylenediamine, propylenediamine, butylenediamine and pentylenediamine;polyalkylene polyamines such as diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine,di(methylethylene)triamine, dibutylenetriamine, tributylenetetramine andpentapentylenehexamine; and piperazine derivatives such asaminoethylpiperazine.

In addition to the above alkenyl or alkylsuccinimide compound, there mayalso be used boron derivatives thereof and/or organic acid-modifiedproducts thereof as the ashless dispersant.

The boron derivatives of the alkenyl- or alkylsuccinimide compound maybe produced by an ordinary method. For example, the boron derivativesmay be produced by first reacting the above polyolefin with maleicanhydride to obtain an alkenylsuccinic anhydride, and then reacting theresulting alkenylsuccinic anhydride with an intermediate productobtained by reacting the above polyamine with a boron compound, such asboron oxide, a boron halide, boric acid, boric anhydride, a boric acidester and an ammonium salt of orthoboric acid, to imidize thealkenylsuccinic anhydride.

The content of boron in the boron derivatives is not particularlylimited, and is preferably in the range of 0.05% to 5% by mass, morepreferably 0.1% to 3% by mass, in terms of boron element.

The compounding amount of the ashless dispersant is preferably 0.5% to15% by mass, more preferably 1% to 10% by mass, still more preferably 3to 7% by mass, based on a total amount of the lubricating oilcomposition.

When the compounding amount is less than 0.5% by mass, the effect onbase number retaining property at high temperatures is small. When thecompounding amount exceeds 15% by mass, the fluidity at low temperaturesof the lubricating oil composition is considerably deteriorated. Thus,either case is not preferable.

As the above-mentioned metallic detergent, there may be used anyalkaline earth metal-based detergents which are employed for ordinarylubricating oils. Examples of the alkaline earth metal-based detergentinclude alkaline earth metal sulfonates, alkaline earth metal phenates,alkaline earth metal salicylates and mixtures of two or more thereof.

As the alkaline earth metal sulfonates, there may be mentioned alkalineearth metal salts of an alkyl aromatic sulfonic acid obtained bysulfonating an alkyl aromatic compound having a molecular weight of 300to 1,500, preferably 400 to 700. Among them, magnesium salts and/orcalcium salts, especially calcium salts are preferred.

As the alkaline earth metal phenates, there may be mentioned alkalineearth metal salts of alkylphenols, alkylphenol sulfides and Mannichreaction products of alkylphenols. Among them, magnesium salts and/orcalcium salts, especially calcium salts are preferred.

As the alkaline earth metal salicylates, there may be mentioned alkalineearth metal salts of alkyl salicylic acids. Among them magnesium saltsand/or calcium salts, especially calcium salts are preferred.

The alkyl group contained in the compounds constituting the abovealkaline earth metal-based detergents is preferably a C₄ to C₃₀ alkylgroup, more preferably a C₆ to C₁₈ linear or branched alkyl group. Thesealkyl groups may be straight chained or branched.

These alkyl groups may be primary alkyl groups, secondary alkyl groupsor tertiary alkyl groups.

As the alkaline earth metal sulfonates, alkaline earth metal phenatesand alkaline earth metal salicylates, there may be used neutral alkalineearth metal sulfonates, neutral alkaline earth metal phenates andneutral alkaline earth metal salicylates which may be produced bydirectly reacting the above alkyl aromatic sulfonic acids, alkylphenols,alkylphenol sulfides, Mannich reaction products of alkylphenols, alkylsalicylic acids or the like with an alkaline earth metal base such as anoxide or a hydroxide of an alkaline earth metal such as magnesium and/orcalcium or which may be produced by once forming an alkali metal saltthereof and then converting the alkali metal salt into an alkaline earthmetal salt. Further, there may also be used basic alkaline earth metalsulfonates, basic alkaline earth metal phenates and basic alkaline earthmetal salicylates which may be produced by heating neutral alkalineearth metal sulfonates, neutral alkaline earth metal phenates andneutral alkaline earth metal salicylates together with an excess amountof an alkaline earth metal salt or an alkaline earth metal base in thepresence of water. Furthermore, there may also be used perbasic alkalineearth metal sulfonates, perbasic alkaline earth metal phenates andperbasic alkaline earth metal salicylates which may be produced byreacting neutral alkaline earth metal sulfonates, neutral alkaline earthmetal phenates and neutral alkaline earth metal salicylates with analkaline earth metal carbonate or an alkaline earth metal borate in thepresence of carbon dioxide.

The metallic detergent used in the present invention is preferably analkaline earth metal salicylate or alkaline earth phenate, especially aperbasic salicylate or perbasic phenate, for reasons of reducing asulfur content of the composition.

The total base number of the metallic detergent used in the presentinvention is preferably 10 to 500 mg KOH/g, more preferably 15 to 450 mgKOH/g. The metallic detergent may be selected from these detergents andused singly or in combination of two or more thereof.

The term “total base number” as used herein means the value as measuredby a potentiometric titration method (base number/perchlorate method)according to the Item 7 of JIS K 2501 “Petroleum Products andLubricants-Neutralization Number Testing Method.”

The metal ratio of the metallic detergent used in the present inventionis not specifically limited. The metallic detergent having a metal ratioof 20 or less may be generally used singly or as a mixture of two ormore thereof. The metallic detergent having a metal ratio of preferably3 or less, more preferably 1.5 or less, still more preferably 1.2 orless, is particularly suitably used for reasons of further improvedoxidation stability, base number retaining property, high-temperaturedetergency, etc.

Meanwhile, the term “metal ratio” as used herein means a ratiorepresented by the formula: (valence of a metal element)×(content (mol%) of the metal element)/(content (mol %) of a soap group) wherein themetal element is calcium, magnesium, etc., and the soap group is asulfonic group, a phenol group, a salicylic group, etc.

The compounding amount of the metallic detergent is preferably 0.01% to20% by mass, more preferably 0.1% to 10% by mass, still more preferably0.5% to 5% by mass, based on the total amount of the lubricating oilcomposition.

A compounding amount of the metallic detergent less than 0.01% by massis not preferable because performances such as high temperaturedetergency, oxidation stability and base number retaining property arenot easily obtainable. When the amount of the metallic detergentcompounded is 20% by mass or less, an effect proportional to thecompounding amount of the metallic detergent may be generally obtained.In spite of the above specified range, however, it is important that theupper limit of the compounding amount of the metallic detergent shouldbe as low as possible. By so doing, the metal content, namely sulfuricacid ash content, of the lubricating oil composition is reduced, withthe result that the exhaust gas purification device of automobiles isprevented from being deteriorated.

The metallic detergent may be used singly or in combination of two ormore thereof as long as the content thereof lies within theabove-specified range.

Specifically, among the above-mentioned metallic detergents, perbasiccalcium salicylate and perbasic calcium phenate are particularlypreferred. Among the above-mentioned ashless dispersants, theabove-mentioned bis-polybutenylsuccinimide is particularly preferred.Meanwhile, it is preferred that perbasic calcium salicylate and perbasiccalcium phenate each have a total base number of 100 to 500 mgKOH/g,more preferably 200 to 500 mgKOH/g.

As the above-mentioned viscosity index improver, there may be mentioned,for example, polymethacrylates, dispersion type polymethacrylates,olefin-based copolymers (such as ethylene-propylene copolymers),dispersion type olefin-based copolymers and styrene-based copolymers(such as styrene-diene copolymers and styrene-isoprene copolymers).

The compounding amount of the viscosity index improver is preferably0.5% to 15% by mass, more preferably 1% to 10% by mass, based on thetotal amount of the lubricating oil composition from the standpoint ofeffects attained by addition thereof.

As the above-mentioned pour point depressant, there may be mentioned,for example, polymethacrylates having a weight-average molecular weightof about 5,000 to about 50,000.

The compounding amount of the pour point depressant is generally 0.1% to2% by mass, more preferably 0.1% to 1% by mass, based on the totalamount of the lubricating oil composition from the standpoint of effectsattained by addition thereof.

As the metal deactivator, there may be mentioned, for example,benzotriazole-based compounds, tolyl triazole-based compounds,thiadiazole-based compounds and imidazole-based compounds.

The compounding amount of the metal deactivator is preferably 0.01% to3% by mass, more preferably 0.01% to 1% by mass, based on the totalamount of the lubricating oil composition.

As the rust inhibitor, there may be mentioned, for example, petroleumsulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates,alkenylsuccinic acid esters and polyhydric alcohol esters.

The compounding amount of the rust inhibitor is preferably 0.01% to 1%by mass, more preferably 0.05% to 0.5% by mass, based on the totalamount of the lubricating oil composition from the standpoint of effectsattained by addition thereof.

As the above-mentioned defoaming agent, there may be mentioned, forexample, silicone oils, fluorosilicone oils and fluoroalkyl ethers. Thecompounding amount of the defoaming agent is preferably 0.005% to 0.5%by mass, more preferably 0.01% to 0.2% by mass, based on the totalamount of the lubricating oil composition from the standpoint of abalance between the defoaming effect and economy.

The lubricating oil composition of the present invention may furthercontain a friction modifier, an anti-wear agent and an extreme pressureagent, if necessary. The friction modifier herein is a compound otherthan the polar group-containing compounds which are an essentialingredient of the present invention. The compounding amount of thefriction modifier agent is preferably 0.01% to 2% by mass, morepreferably 0.01% to 1% by mass or less, based on the total amount of thelubricating oil composition.

As the anti-wear agent or the extreme-pressure agent, there may bementioned sulfur containing compounds such as zinc dithiophosphate, zincphosphate, zinc dithiocarbamate, molybdenum dithiocarbamate, molybdenumdithiophosphate, disulfides (other than the sulfur-containing compoundsof the general formula (I) or (II) used in the present invention;dibenzyldisulfide is an example thereof), sulfurized olefins, sulfurizedoils and fats, sulfurized esters, thiocarbonates, thiocarbamates andpolysulfides; phosphorus containing compounds such as phosphorous acidesters, phosphoric acid esters, phosphonic acid esters and amine saltsor metal salts of these esters; and sulfur- and phosphorus-containinganti-wear agents such as thiophosphorous acid esters, thiophosphoricacid esters, thiophosphonic acid esters and amine salts or metal saltsof these esters.

The compounding amount of the anti-wear agent or the extreme-pressureagent to be compounded should be such that the phosphorus content,sulfur content and metal content of the lubricating oil are notexcessively large by addition thereof.

The lubricating oil composition of the present invention may beformulated as described in the foregoing and preferably has thefollowing properties:

(1) the sulfuric acid ash content (JIS K 2272) is 0.6% by mass or less,more preferably 0.1% by mass or less; and

(2) the phosphorus content (JPI-5S-38-92) is 0.5% by mass or less, morepreferably 0% by mass.

Additionally, it is more preferred that the following properties aremet:

(3) the sulfur content (JIS K 2541) is 0.4% by mass or less, morepreferably 0.2% by mass or less; and

(4) the boron content is 0.4% by mass or less, more preferably 0.2% bymass or less.

The lubricating oil composition of the present which satisfies the aboveproperties can suppress deterioration of an oxidation catalyst, a threeway catalyst, an NOx occlusion reduction catalyst, a diesel particulatefilter (DPF), etc. which are used in automobile engines.

The lubricating oil composition of the present invention uses acombination of the above-described sulfur-containing compound and thepolar group-containing compound. As a result of such combined use, thereare achieved wear resisting and friction reducing effects which are farsuperior to those attained by separate use thereof. Accordingly, evenwhen zinc dithiophosphate which has been hitherto often used as alubricant oil additive is not used, the lubricating oil compositionshows sufficiently excellent lubricating performance and makes itpossible to achieve properties of low sulfuric acid ash, etc.Furthermore, the lubricating oil composition of the present inventionexhibits excellent friction reducing effect even when used for aDLC-treated sliding part as described hereinafter.

The lubricating oil composition of the present invention can be suitablyused as a lubricant oil for use in an internal combustion engine, suchas a gasoline engine, a diesel engine or a gas engine, for two-wheeledvehicles, four-wheeled vehicles, power generators, ships or the like,and is particularly suited for internal combustion engines equipped withan exhaust gas purification device because of its low phosphoruscontent, low sulfur content and low sulfuric acid ash content.

The lubricating oil composition of the present invention is alsosuitably used for applications other than those described above.Especially, since the lubricating oil composition of the presentinvention shows excellent wear resistance and friction reducing effect,it can be used for lubrication of internal combustion engines, automatictransmissions, continuously variable transmissions, manualtransmissions, power steerings, shock absorbers, compressors, coolingmedium compressors, refrigerators, hydraulic pumps and clutch pulleys.Namely, the lubricating oil composition of the present invention may beused as internal combustion engine oils, automatic transmission oils,continuously variable transmission oils, manual transmission oils, powersteering oils, shock absorber oils, compressor oils, refrigerator oils,hydraulic pump oils and clutch pulley lubricating oils and greases.

The lubricating oil composition of the present invention exhibitsfriction reducing effect and excellent wear resistance not only for asliding surface of a metal such as a steel but also for a slidingsurface having a DLC film on at least a portion thereof.

It is preferred that the hydrogen content of such a DLC be 40% by atomor less, more preferably 30% by atom or less, particularly preferably20% by atom or less.

A counter member with which the sliding surface of such a DLCfilm-bearing member is to be brought into contact is not specificallylimited and may be, for example, an iron or iron alloy member, aluminumalloy member or an organic material such as a resin or rubber material.

EXAMPLES

The present invention will be next described in more detail by way ofExamples and Comparative Examples. The scope of the present invention,however, is not limited to these examples in any way.

Methods for Measuring Properties and Performances:

The properties and performances of the lubricating oil compositionsobtained in the following Examples and Comparative Examples are measuredby the methods shown below.

(1) Kinematic Viscosity:

Measured according to JIS K 2283.

(2) Phosphorus Content:

Measured according to JPI-5S-38-92.

(3) Sulfur Content:

Measured according to JIS K 2541.

(4) Boron Content:

Measured according to JPI-5S-38-92.

(5) Sulfuric Acid Ash Content:

Measured according to JIS K 2272.

(6) Reciprocating Friction Test

The test was carried out with a reciprocating friction tester todetermine a friction coefficient using a test plate and a test ballshown below under the conditions shown below.

Test plate: SUJ-2 plateTest ball: SUJ-2 ball (½ in) treated with DLC (hydrogen content: 20%)

—Test Conditions—

Testing temperature: 100° C.

Load: 200 g

Amplitude: 10 mm

Sliding speed: 1.0 mm/sec

(7) Frictional Wear Test

The test was carried out with a reciprocating friction tester todetermine wear track length using a test plate and a test ball shownbelow under the conditions shown below.

Test plate: SUJ-2 plateTest ball: SUJ-2 ball (10 mm diameter)

—Test Conditions—

Testing temperature: 100° C.

Load: 200 N

Amplitude: 10 mm

Frequency: 10 Hz

Testing time: 30 min

Examples 1 to 7 and Comparative Examples 1 to 9

The base oil and additives shown in Table 1 were blended in theproportion shown in Table 1 to prepare lubricating oil compositions. Theproperties, formulations and performances of the compositions are alsoshown in Table 1.

TABLE 1 Example 1 2 3 4 5 6 7 Formulation Base oil 88.60 88.60 88.6088.60 88.60 88.60 87.80 Composition Sulfur-containing compound A 0.400.40 0.40 0.40 0.40 0.40 — (% by mass) Sulfur-containing compound B — —— — — — 1.20 Polar group-containing compound A 0.50 — — — — — 0.50 Polargroup-containing compound B — 0.50 — — — — — Polar group-containingcompound C — — 0.50 — — — — Polar group-containing compound D — — — 0.50— — — Polar group-containing compound E — — — — 0.50 — — Polargroup-containing compound F — — — — — 0.50 — Viscosity index improver5.00 5.00 5.00 5.00 5.00 5.00 5.00 Pour point depressant 0.30 0.30 0.300.30 0.30 0.30 0.30 Polybutenylsuccinimide 4.00 4.00 4.00 4.00 4.00 4.004.00 Phenol-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50Amine-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Zincdialkyldithiophosphate — — — — — — — Etc. 0.20 0.20 0.20 0.20 0.20 0.200.20 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 PropertiesPhosphorus content (% by mass) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sulfurcontent (% by mass) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Boron content (% bymass) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 Sulfuric acid ash content (% bymass) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Reciprocating friction test(friction coefficient) 0.115 0.117 0.113 0.110 0.116 0.117 0.114Frictional wear test (wear track diameter; mm) 0.46 0.45 0.45 0.44 0.420.43 0.45

TABLE 2 Comparative Example 1 2 3 4 5 6 7 8 9 Formulation Base oil 89.1089.00 89.00 89.00 89.00 89.00 89.00 88.40 87.90 CompositionSulfur-containing 0.40 — — — — — — — — (% by mass) compound A Polargroup-containing — 0.50 — — — — — — — compound A Polar group-containing— — 0.50 — — — — — — compound B Polar group-containing — — — 0.50 — — —— — compound C Polar group-containing — — — — 0.50 — — — — compound DPolar group-containing — — — — — 0.50 — — — compound E Polargroup-containing — — — — — — 0.50 0.50 0.50 compound F Viscosity indeximprover 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Pour pointdepressant 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30Polybutenylsuccinimide 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00Phenol-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50Amine-based antioxidant 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50Zinc — — — — — — — 0.60 0.60 dialkyldithiophosphate Etc. 0.20 0.20 0.200.20 0.20 0.20 0.20 0.20 0.20 Total 100.00 100.00 100.00 100.00 100.00100.00 100.00 100.00 100.00 Properties Phosphorus content 0.00 0.00 0.000.00 0.00 0.00 0.00 0.05 0.05 (% by mass) Sulfur content (% by mass) 0.10.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 Boron content (% by mass) 0.08 0.08 0.080.08 0.08 0.08 0.08 0.08 0.08 Sulfuric acid ash content 0.05 0.05 0.050.05 0.05 0.05 0.05 0.17 0.17 (% by mass) Reciprocating friction test0.140 0.125 0.128 0.124 0.125 0.126 0.124 0.155 0.143 (frictioncoefficient) Frictional wear test (wear track diameter; 0.52 0.64 0.650.63 0.63 0.60 0.61 0.48 0.50 mm) Note: Base oil: Hydrogenated refinedbase oil (kinematic viscosity at 40° C.: 21 mm²/s; kinematic viscosityat 100° C.: 4.5 mm²/s; viscosity index: 127; % C_(A): 0.0; sulfurcontent: less than 20 ppm by mass; NOACK test evaporation amount: 13.3%by mass) Sulfur-containing compound A: 1,1-bis(octoxycarbonylmethyl)disulfide Sulfur-containing compound B: Tetra-1-hexyl dithiomalate Polargroup-containing compound A: Glycerol monoolate Polar group-containingcompound B: Oleic acid diethanolamide Polar group-containing compound C:Glycerol monooleyl ether Polar group-containing compound D:N,N-Dipolyoxyethylene-N-oleylamine Polar group-containing compound E:Reaction product between glycerol monoolate and boric acid Polargroup-containing compound F: Reaction product between oleic aciddiethanolamide and boric acid Viscosity index improver: Polymethacrylate(weight average molecular weight: 420,000; resin content: 39% by mass)Pour point depressant: Polyalkyl methacrylate (weight average molecularweight: 6,000) Polybutenylsuccinimide: Number-average molecular weightof polybutenyl group: 1,000; Nitrogen content: 1.76% by mass; Boroncontent: 1.9% by mass Phenol-based antioxidant: Octadecyl3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate Amine-based antioxidant:Dialkyldiphenylamine (nitrogen content: 4.62% by mass) Zincdialkyldithiophosphate (Zn content: 9.0% by mass, phosphorus content:8.2% by mass, sulfur content: 17.1% by mass, alkyl group: mixture ofsecondary butyl group and secondary hexyl group) Etc.: Defoaming agentand metal deactivator

As shown in Tables 1 and 2, the lubricating oil compositions of Examples1 to 7 gives low friction coefficients and small wear track diametersdue to the synergetic effect of the combined use of thesulfur-containing compound and the polar group-containing compound.

Namely, in Comparative Example 1 which does not use a polargroup-containing compound, the friction coefficient is 0.140. InComparative Examples 2 to 7 which do not use a sulfur-containingcompound, the friction coefficient is 0.124 to 0.128. However, whenthese compounds are used in combination, the friction coefficient can bereduced to 0.110 to 0.117 (Examples 1 to 7). Similarly, althoughComparative Example 1 which does not use a polar group-containingcompound gives a wear track diameter of 0.52 and Comparative Examples 2to 7 which do not use a sulfur-containing compound give a wear trackdiameter of 0.60 to 0.65, the wear track diameter can be reduced to 0.42to 0.46 by using these compounds in combination (Examples 1 to 7).

Also, it will be appreciated by comparison between Examples 1 to 7 andComparative Example 8 that the effect achieved by the combined use ofthe additives according to the present invention is superior to theeffect attained by using zinc dialkyldithiophosphate alone. Further, aswill be appreciated from the results in Comparative Example 9, when thezinc dialkyldithiophosphate is substituted for the sulfur-containingcompound of the present invention and is used in combination with thepolar group-containing compound, it is impossible to obtain such a lowfriction coefficient and a small wear track diameter as attained inExamples 1 to 7.

As described in the foregoing, the combined use of the specificsulfur-containing compound and the specific polar group-containingcompound can make it possible to obtain higher wear resistance than thatattained by the zinc dialkyldithiophosphate. Consequently, it ispossible to provide a lubricating oil composition which is excellent inwear resistance, despite its low phosphorus content, low sulfur contentand low sulfuric acid ash content, and which exhibits excellent frictionreducing effect.

INDUSTRIAL APPLICABILITY

According to the present invention there is provided a lubricating oilcomposition which is excellent in wear resistance, despite its lowphosphorus content, low sulfur content and low sulfuric acid ashcontent, and which exhibits excellent friction reducing effect even whenused for a DLC-treated sliding part. The lubricating oil compositionaccording to the present invention, therefore, can be particularlysuitably used as a lubricating oil composition for internal combustionengines such as gasoline engines, diesel engines and gas engines.

1. A lubricating oil composition, comprising: (A) a base oil; (B) atleast one selected from the group consisting of a sulfur-comprisingcompound of formula (I) and formula (II):

wherein: R¹ to R¹² are each independently is a hydrogen atom; ahydrocarbon group selected from the group consisting of an alkyl group,a cycloalkyl group, an alkenyl group, a cycloalkenyl group, and an arylgroup; or a hetero atom-comprising group comprising an atom selectedfrom an oxygen atom, a nitrogen atom, and a sulfur atom, which iscontained a hydrocarbon group selected from the group consisting of analkyl group, a cycloalkyl group, an alkenyl group, a cycloalkenyl group,and an aryl group; each Y is independently a divalent group selectedfrom the group consisting of —O—, —S—, —SO—, —SO₂—, —(C═O)O—, —(C═O)NH—,—O(C═O)NH—, —C(═O)—, —N(H)—, —NHCONH—, —N═N—, —NH—C(═NH)—NH—, —S—C(═O)—,—NH—C(═S)—, and —NH—C(═S)—NH—; x is an integer from 1 to 3; and each nis independently an integer from 1 to 5; and (C) a polargroup-comprising compound comprising at least one polar group selectedfrom the group consisting of an amino group, an amide group, and ahydroxyl group, and which comprises a C₃ to C₂₄ alkyl group.
 2. Thecomposition of claim 1, wherein compound (C) is at least one selectedfrom the group consisting of a glycerol partial ester of a fatty acid, aglycerol monoether compound, an amine compound, and an amide compound.3. The composition of claim 1, wherein compound (C) is a glycerolmonoester of a fatty acid represented by the general formula (III) or(IV):

wherein each R¹³ is independently a C₃ to C₂₄ alkyl group, or a glycerolmonoether compound of formula (V) or (VI):

wherein each R¹⁴ is independently a C₃ to C₂₄ alkyl group.
 4. Thecomposition of claim 1, wherein the compound (C) is an amine compound offormula (VII):

wherein R¹⁵ is a C₃ to C₂₄ alkyl group and each R¹⁶ is independently ahydrogen atom or a group comprising a hydroxyl group substituted for aterminal hydrogen atom of a straight chained C₂ to C₄ alkyl group, or anamide compound of formula (VIII):

wherein is a C₃ to C₂₄ alkyl group, and each R¹⁸ is independently ahydrogen atom or a group comprising a hydroxyl group substituted for aterminal hydrogen atom of a straight chained C₂ to C₄ alkyl group. 5.The composition of claim 1, comprising a phosphorus content of 0.5% bymass or less and a sulfuric acid ash content of 0.6% by mass or less,each based on a total mass of the composition.
 6. The composition ofclaim 1, comprising a phosphorus content of 0% by mass and a sulfuricacid ash content of 0.1% by mass or less, each based on a total mass ofthe composition.
 7. The composition of claim 1, being adapted for asliding part which is treated with diamond-like carbon (DLC).
 8. Thecomposition of claim 1, wherein the base oil (A) has a kinematicviscosity at 100° C. of 2 to 30 mm²/s.
 9. The composition of claim 1,wherein the base oil (A) has a viscosity index of 70 or more.
 10. Thecomposition of claim 1, comprising 0.01% to 5.0% by mass of the compound(B), based on a total mass of the composition.
 11. The composition ofclaim 1, comprising 0.1% to 2.0% by mass of the compound (B), based on atotal mass of the composition.
 12. The composition of claim 1,comprising 0.01% to 5.0% by mass of the compound (C), based on a totalmass of the composition.
 13. The composition of claim 1, comprising 0.1%to 2.0% by mass of the compound (C), based on a total mass of thecomposition.
 14. The composition of claim 1, wherein the compound (B) offormula (I) is at least one selected from the group consistingbis(methoxycarbonylmethyl)disulfide, bis(ethoxycarbonylmethyl)disulfide,bis(n-propoxycarbonylmethyl)disulfide,bis(isopropoxycarbonylmethyl)disulfide,bis(n-butoxycarbonylmethyl)disulfide,bis(n-octoxycarbonylmethyl)disulfide,bis(n-dodecyloxycarbonylmethyl)disulfide,bis(cyclopropoxycarbonylmethyl)disulfide,1,1-bis(2-methoxycarbonylethyl)disulfide,1,1-bis(3-methoxycarbonyl-n-propyl)disulfide,1,1-bis(4-methoxycarbonyl-n-butyl)disulfide,1,1-bis(2-ethoxycarbonylethyl)disulfide,1,1-bis(2-n-propoxycarbonylethyl)disulfide,1,1-bis(2-isopropoxycarbonylethyl)disulfide, and1,1-bis(2-cyclopropoxycarbonylethyl)disulfide.
 15. The composition ofclaim 1, wherein the compound (B) of formula (II) is at least oneselected from the group consisting of tetramethyl dithiomalate,tetraethyl dithiomalate, tetra-1-propyl dithiomalate, tetra-2-propyldithiomalate, tetra-1-butyl dithiomalate, tetra-2-butyl dithiomalate,tetraisobutyl dithiomalate, tetra-1-hexyl dithiomalate, tetra-1-octyldithiomalate, tetra-1-(2-ethyl)hexyl dithiomalate,tetra-1-(3,5,5-trimethyl)hexyl dithiomalate, tetra-1-decyl dithiomalate,tetra-1-dodecyl dithiomalate, tetra-1-hexadecyl dithiomalate,tetra-1-octadecyl dithiomalate, tetrabenzyl dithiomalate,tetra-α-(methyl)benzyl dithiomalate, tetra-α,α-dimethylbenzyldithiomalate, tetra-1-(2-methoxy)ethyl dithiomalate,tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxy)ethyldithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate,tetra-1-(2-butoxybutoxy)ethyl dithiomalate and tetra-1-(2-phenoxy)ethyldithiomalate.